This invention relates to optically- and electron-beam-pumped solid-state lasers.
The basic problem impeding the efficient operation of optically-pumped solid-state lasers is that the typical solid-state laser materials consist of a wide bandgap insulating host material doped with optically active impurity atoms. The impurities are typically either rare-earth ions (Nd.sup.3+, Er.sup.3+) or the transition metals ions (Cr.sup.3+, Ti.sup.3+). The absorption spectrum of such ions is characterized by the lines associated with the transitions between the shielded (and thus narrow) f or d atomic levels. However, most of the pump sources for such lasers, such as high pressure gas discharge or incandescent lamps, are characterized by their extremely wide emission spectrum. Therefore, only a small percentage of the pumped light is actually absorbed by the laser material. For a small diameter laser rod, this is typically less than 10%. As a result, the flashlamp pumped solid-state laser usually requires a bulky power supply and a water cooled system. Besides inefficiency, this renders the laser system useless for applications where portability is a key requirement.
In recent years, the laser diode has emerged as a promising alternative to flashlamp pumping of solid-state lasers. The high pumping efficiency, compared to flashlamps, stems from the better spectral match between the laser-diode emission and the rare-earth absorption bands. As a result, the thermal load on both the laser rod and the pump is reduced. The system weight and power consumption are also substantially reduced with increased reliability. However, the cost of the diode laser arrays makes it expensive. In addition, the laser diodes require high current power supplies that are usually heavy rendering the lasers impractical for airborne and space applications. Therefore, scientists have been trying to harness an alternative energy pump source, the most efficient of which is solar energy.
Considerable research has been done on solar-pumped solid-state lasers such as Nd:glass [1], Nd:CaWO.sub.4 [2] and Nd:YAG [3]. [Bracketed reference numbers are identified in the annexed Appendix]. In the latter case, a solar pumped Nd:Yag laser has produced up to 5 W of continuous wave (CW) output power. The basic geometry of the solar-pumped laser consists of a Cassegranian telescope to collect the solar radiation and suitable optics to concentrate the pumped light onto the face of the end-pumped laser rod [3]. More recently, some non-imaging solar concentrators have been proposed.
Typically, the diameter of the collecting optics must exceed 1 m in order to achieve the lasing threshold. This fact severely limits the performance of the solid-state lasers in those applications where the size of the payload is crucial. The underlying reason for such a "hunger" for the sunlight, is the same one that limits the performance of the flashlamp-pumped lasers; the mismatch between the line absorption spectrum of the laser material and the continuous emission spectrum of the pump source--the sun. Numerous attempts to enhance the transfer of the pump source radiation to the laser emission have succeeded in the development of the so-called "sensitized" lasers, such as Cr:Nd:GSGG [4]. In this material, the Nd.sup.3+ is the active ion and the Cr.sup.3+ is a "sensitizer", which has a large absorption band over a wide spectral range. The excitation absorbed by the broad Cr.sup.3+ absorption bands is transferred to the Nd.sup.3+ ions. This process is reasonably efficient in the GSGG host, but this is a rare coincidence. In many other hosts, like YAG, the energy transfer is quite inefficient, since transitions between the "sensitizer" and active ion is spin forbidden. Given the difficulties encountered in the production of GSGG, this scheme has not found many applications.